Most notably chirons.
Arthrons with launchers are annoying, but you can usually shoot them in their arms and it's not a major problem.
But Chirons apply a ridiculous amount of acid per each blob that hits, and if they shoot a tight cluster of acid blobs you can have squaddies getting 300-500 acid applied from one unseen attacker that is so far on the other side of the map there is no way to reasonably take care of before they get to attack.
Which means on turn 2, since there is no way to remove acid or otherwise mitigate the damage, outside of maybe a level 7 technician with boatloads of will, you have to extract anyone hit, or they are all dead before turn 3.
This is the biggest BS acid issue.